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How to use AC to power DC LED

  1. Nov 4, 2016 #1
    I need guidance in the explanation of utilizing LED light bars for a different function then designed. I do stage production, sound and lights for concerts and would like to adapt these LED light bars to "blind" to crowd.

    My issue is that each light bar operates 12-30 Volts DC @ 300 Watts. In order to power and control with our DMX software (standard lighting control language/interface for stage production) I would need to build a more complicated device for each light bar. Meaning I would need DMX control PC board, relay to handle high wattage, power source, XLR in/out jack for communication, IEC jack, fuses, housing etc... Multiply by 4 costs add up FAST.

    My thoughts include:

    A single high wattage (24volt @1500watt) DC power supply to power all.

    Then I thought, can I wire in series to 120AC, this would put me at 30volts AC for each fixture. This would make controlling ALOT easier and cheaper because I could use an off the shelf DMX relay switch to control the bars.

    I hope I explained well...

    I am looking for any suggestions.

    Feel free to contact me at anytime: <<personal details removed>>
    outY0.jpg Benson-1.jpg
    Last edited by a moderator: Nov 4, 2016
  2. jcsd
  3. Nov 4, 2016 #2

    Simon Bridge

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    Welcome to PF - I suspect it also makes things more dangerous.
    Depending how the leds are wired, you may need at least half-wave rectification to avoid destroying the lamps when the current reverses direction. Why not use an old-style PSU?
  4. Nov 4, 2016 #3
    How would I remove one of the waves?
    What is PSU?

    I have a feeling I may be in diapers, compared to most in here... Perhaps pull ups... haha!
  5. Nov 4, 2016 #4

    Simon Bridge

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    PSU = Power Supply Unit - it is the box on your laptop power cord.
    The AC wave is a single wave of current - when the wave goes from positive to negative values (or the other way) it means that the current changes direction.
    LEDs have to have current going only in one direction or they don't work (or blow up if the voltage is too high).

    You can make the wave only go in one direction by using a device called a "half wave rectifier" or a "bridge rectifier" - the second is better but more expensive.
    These devices use high power diodes in a configuration that forces the current coming out to only go in one direction - though it still varies.
    Look up the names.

    PSU's can be purchased as boxes from electronics stores or constructed from common simple components.

    If you do not know about these things - it is not a good idea to be messing about with mains powered circuits - you can easily kill someone.
  6. Nov 4, 2016 #5


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    To convert that AC to something better resembling DC you would need a rectifier. But even then it would be a pulsating voltage sweeping from 0v up to 42v and with the light bars designed for 12-30v DC they may not be happy with this.

    Placing all 3 light bars in jeopardy is probably not a risk worth taking.
  7. Nov 4, 2016 #6

    Simon Bridge

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  8. Nov 4, 2016 #7


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    Another thing to keep in mind is that the line voltage is specified in RMS, not peak value. In modern times the standard nominal value for AC voltage across North America is 120 V and is supposed to be maintained to within +/- 5%. Thus it can vary from time to time and place to place depending upon local grid circumstances from, say, 114 V to 126 V RMS, meaning that the peak values can range from 161 V to 178 V. So what you thought was a nice division of 120 V by 4 to yield 30 V per unit actually could end up delivering between 40 V and 45 V, with 42.4 V as the nominal value. That would be the voltage peaks. The average voltage delivered would be much lower (peaks make up a tiny part of the AC cycle).

    To judge how the individual units would fare under those conditions we'd need to know more about their internal wiring and the LED device specs. We have no idea what insulation or grounding precautions have been observed for these units if they are intended to be run on low voltage DC; It would not surprise me to learn that one supply connection is directly wired to the metal enclosure.

    As a practical solution, placing several loads which are expecting low voltage DC in series with an AC source has too many issues of safety and unknown behavior to be recommended.
  9. Nov 4, 2016 #8

    jim hardy

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    i'd inquire of some manufacturers. That way you can buy something that's designed and built safe .

    These guys make controllable LED supplies. Try searching on some keywords from ther literature, or give them a call.

    these guys are another source


    We generally don't encourage beginners to undertake DIY projects involving mains power . It's too easy to unwittingly build something that's dangerous to tiny, curious fingers..

    old jim
  10. Nov 4, 2016 #9


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    If you wire all 4 in series then they will all be ON or OFF at the same time. If that's OK then you can wire them in parallel and use one 12-30 Volts DC 1200W controller rather than 4 separate 300W controllers.
  11. Nov 4, 2016 #10
    Do you know of anything that could be substituted for a standard PSU? I am trying to get around spending $300-$500 on a 24vdc 1200watt PSU. i.e. Battery charger, welder, or perhaps removing a PSU from a cheaper piece of equipment.
  12. Nov 4, 2016 #11

    jim hardy

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    You didn't say whether you need adjustable voltage or just on-off.

    300 watts at 30 volts is only ten amps and 0.4 horsepower . 1200 watts is 40 amps and 1.6 horsepower.

    Speaking as a hobbyist here not an engineer
    and you seem a practical sort
    two thoughts.
    1. Battery charger:
    If you used battery chargers your controls would be on the safe side of their isolation transformer. And you'd be dealing with low voltage .
    Many Golf Cart battery chargers are 36 volts, mine was only 25 amps though.
    Golfcart chargers are pricey new but you can probably find a used one that needs just minor repair. When the plug falls off and the meter gets smashed some folks throw them out. Inquire at your local golf cart dealer or golf course. I would get an old fashioned one that's transformer based not switchmode, you can tell by weight the transformer one will be at least 5X heavier.
    I see them often at my local scrap metal salvage yard .The sell for scrap price of 30 cents a pound and weigh maybe fifty pounds.
    You'd probably want to add 'filter capacitors ' count on about seventy five bucks for enough industrial grade ones.

    2. Another option would be rotating machinery.
    A modern automobile alternator is capable of typically eighty amps and if it's not got an internal regulator would easily make thirty volts(i've seen them make 150 volts).
    My 1995 Dodge minivan's alternator had no internal regulator its field was controlled externally by the ECU. So it would be a candidate.
    Such an alternator driven by a 2hp AC motor should do the job. You'd adjust voltage by controlling field current. You'll have to experiment with pulley size to make 40 amps at 30 volts instead of 12 with reasonable field current. Don't allow more than 12 volts across the field i'd be more comfortable with 6.
    That has the advantage of adjustable voltage.

    Above is just the musings of a tinkerer. I never built one.

    If you dont need adjustable voltage
    Something like this might be worth an experiment

    again, you'll work on its safe side.

    old jim
  13. Nov 4, 2016 #12


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    Many of those light bars have constant current driver circuitry inside. Note the 12-30V range. Running them in series may not work very well if at all. They also come in 120VAC versions that tend to be cheaper.

  14. Nov 5, 2016 #13

    jim hardy

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    Wow i missed that fine point ! Assumed that voltage range was for dimming .

    good eye, Bob .
  15. Nov 6, 2016 #14
    In my hobby of powered models there's a need for inexpensive high power supplies to charge large capacity batteries quickly. A common solution is to adapt a second hand computer server power supply. That might be a cheap way for you to resolve your issue. You can buy the supplies ready to stack to achieve the required voltage, two in series will provide 24V which may be within your needs. They're quite inexpensive and the quality/safety is top notch coming from makers like HP and Dell. Here's a link.
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